There are tons of different voice over IP programs, some of the most popular ones include Ventrilo and Teamspeak 2. Here is some information that may help you decide which one you'd like to use:
The most popular VoIP in Guild Wars and other games. You can run a free 8-slot server on your own computer. Lots of channel options. Password protected and Text to Speech.Great sound quality.
Uses a lot bandwidth and there a less admin options.
The second most popular VoIP in Guild Wars and most players own TS2. You can run a limited server on your own computer. Channel options and password protected. Great sound quality.
Lower sound quality than ventrilo and may give static if majority of slots are filled.
This is a free to use no limit VoIP. There are a lot of Guild Wars players that have X-Fire installed.
There are no channel options and little admin options. You can not purchase upgrades for you channel.You cannot use press-to-speak in X-Fire.
Avg. Price per Slot (USD)
As you can see the average Teamspeak 2 slot price is much lower than Ventrilo. Teamspeak 2 10-Slot servers prices can go as low as $2.60 per month and Ventrilo 10-Slot servers prices can go as low as $2.69 per month. You just have to register for X-Fire VoIP, but there are a lot of limitations that make it not as useful as Teamspeak 2 or Ventrilo. They can also be extremely expensive per month. You should do some browsing and find the most suitable price that will help you get that VoIP server for your guild.